Re: [PATCH v7 3/3] drm: Add GUD USB Display driver

[Peter Stuge <peter@stuge.se>]

Ilia Mirkin wrote:
> XRGB8888 means that the memory layout should match a 32-bit integer,
> stored as LE, with the low bits being B, next bits being G, etc. This
> translates to byte 0 = B, byte 1 = G, etc. If you're on a BE system,
> and you're handed a XRGB8888 buffer, it still expects that byte 0 = B,
> etc (except as I outlined, some drivers which are from before these
> formats were a thing, sort of do their own thing). Thankfully this is
> equivalent to BGRX8888 (big-endian packing), so you can just munge the
> format.

I understand! Thanks a lot for clarifying.

It makes much more sense to me that the format indeed describes
what is in memory rather than how pixels look to software.


> > > I'm not sure why you guys were talking about BE in the first place,
> >
> > I was worried that the translation didn't consider endianess.
> 
> The translation in gud_xrgb8888_to_color definitely seems suspect.

So to me this means that the gud_pipe translations from XRGB8888 to the
1-bit formats *do* have to adjust for the reversed order on BE.


> There's also a gud_is_big_endian, but I'm guessing this applies to the
> downstream device rather than the host system.

gud_is_big_endian() is a static bool wrapper around defined(__BIG_ENDIAN)
so yes, it applies to the host.

With memory layout being constant I again think gud_xrgb8888_to_color()
needs to take further steps to work correctly also on BE hosts. (Maybe
that's le32_to_cpu(*pix32), maybe drm_fb_swab(), maybe something else?)


> I didn't check if dev->mode_config.quirk_addfb_prefer_host_byte_order
> is set

I can't tell if that's helpful, probably Noralf can.


Thanks a lot

//Peter